Jennifer Selter is known for her "fitspo" (fitness inspiration) photos on Instagram, intended to encourage others to work out. / Eric Levin

by Hilary Sheinbaum, Special for USA TODAY

by Hilary Sheinbaum, Special for USA TODAY

If New Year's resolutions are any indication, Americans yearn to be more physically fit. And many are finding encouragement by getting fitness inspiration - also known as "fitspo" - right in the palm of their hand.

On Instagram, the free app whose 150 million active users share, "like" and comment on 55 million photos and videos each day, "fitspo" and "fitspiration" have become synonymous with images of buff biceps, butts, bellies and breasts - sometimes all in one frame. Many feature beautiful faces, too. Often, they're of amateur fitness fanatics, models, personal trainers and brand-endorsed spokespeople who provide workout tips, clever slogans intended to encourage the masses, and the hope that hard work in the gym will yield desired results.

With each upload, the community and following expands - and some fitspo personalities are becoming stars. Jen Selter, a 20-year-old New Yorker with almost 1.4 million followers, gets roughly 5,000 new ones with every new photo she posts.

"I just want to show people you can stay fit your whole life," says Laura Gordon, 46, formerly a real estate agent in Charlotte, N.C. "Anyone can do it if I can." Also known as @cttchickentuna, Gordon, with more than 323,000 followers, says she spends eight hours exercising and nine hours on Instagram each week.

"I looked at fitspo every night before bed and every morning when I was getting in shape," says Bo Sellers, 28, an aspiring comedian in Los Angeles, who says she lost 50 pounds over the summer. "It was a very big part of my regimen. It kept me going."

But not everyone thinks fitspo is helpful, or healthy.

"We have a world where everybody is so obese, but I don't know if this is going to help matters, because it's so difficult to reach this level," says physician Coral Arvon, director of behavioral health and wellness at Pritikin Longevity Center + Spa in Miami. "It's almost anorexia with just a little bit of muscle over it," she says, referring to fitspo images she's seen online.

Comments on fitspo photos range from support to concern to verbal abuse. Gordon estimates she's blocked 20,000 trash-talking commenters. "There's so much hate," she says. "I'll block 25 to 50 people every time I post a picture."

Women aren't the only ones being clicked on. Joey Swoll, 29, (@joeyswoll) director of sales/head athlete at SHREDZ, a supplement company, says the male-to-female ratio of his more than 177,000 followers is "pretty balanced."

Sergio Rizzuto, 22, a student at Villanova (Pa.) University, operates the @instafitsociety account, with over 593,000 followers. "I originally made the account for guys and girls, but it seemed to get a better response from feminine pictures," says Rizzuto, who spends more than 40 hours a week on Instagram, and says for many it's a full-time job.

"When you post a fitness-related product to (hundreds of thousands) of fitness-interested people, it's perfect. You can post an ad on TV for fitness, but maybe almost 1 out of 10 people will be interested in it. In this, 90% of the people are going to be into it," Rizzuto says.

Massiel Arias, 25, a certified personal trainer in Jersey City, says Nike, Under Armour and New Balance supply her with athletic apparel, but she wears their gear without payment or direct mentions. Her @mankofit account has more than 871,000 followers. Other companies offer Arias $500 to $1,500 for promotional posts, she says, but she declines, because she says her Instagram account is for documenting her method of coping with depression, not ad sales.

Instagram's terms of service do not explicitly ban advertising; it does, however, prohibit partially nude and sexually suggestive content.

Despite the rules, users share content featuring provocative poses, and of course, posters choose their most flattering images. Filters and other enhancements are common,and some people even use professional photographers.

"Whatever is driving all this - commerce, sexuality, comparing (oneself to others) - the reality is that none of that's new," says Robert Weiss, a licensed clinical social worker at Elements Behavioral Health in Long Beach, Calif., who specializes in compulsive sexual behavior online. "It's just the avenue to be able to do it on such a large scale, and the ability to do it yourself (rather) than having to be published in a magazine or be on TV, is new."

And some fitspo posters say they feel a responsibility to their followers.

"They want to have what you have," says Arias. "When you have this type of following, you're responsible for everything you say."

Selter started noticing fitspo at age 15 or 16, and "it motivated me to work out and look like that," says the Long Island native.

A few years later, life imitated art. "I started taking pictures with bright colors; flattering pictures that I know people like seeing. I would post them (online), and my pictures would somehow get on Instagram." Some fitspo accounts featured her images without asking, she says.

Selter activated her own Instagram account in March 2012. Since then, her photo backdrops have included the gym, scenes around New York City, the pool and bed.

Selter typically poses in yoga pants and a sports bra, revealing a slim frame and six-pack abs. Professional photos with retouching, makeup and filters are used in moderation, but much commotion (and some sexually explicit comments) surround her backside, which she says has not been cosmetically enhanced.

She says she's taking classes toward getting her personal training certification, and she wants to create workout videos and diet plans, and have her own line of gym apparel. She's already a paid spokeswoman for the nutritional supplement Game Plan, and says she's been receiving solicitations from other big-name companies as well. Her mom and dad are proud of her, she adds.

"It's not even about numbers anymore," says Selter. "I never expected a million (followers). If I can keep going up, why not? But now's the time to do more than just Instagram, because Instagram is nothing - it's just a social media thing."

â?¢ Celebs known for their derrieres: Kim Kardashian (@kimkardashian: 12 million followers on Instagram) and Jennifer Lopez (@jlo: 941,500 followers)

â?¢ Her role model, fitness guru Jillian Michaels (@jillianmichaels has 57,500 followers on Instagram. Compare that with Selter's backup Instagram account (@jenlselter) which has has more than 56,000 followers.